The delta strain is spreading rapidly across the U.S. CNN reports that to help stem the spread, the Centers for Disease Control and Prevention (CDC) is recommending that people who have been vaccinated return to wearing face masks indoors when in high-risk areas.

According to the CDC, this is especially important in cities where Covid-19 transmission is high. (The CDC has an online tracker that allows anyone to see the level of infection in their area.)

The delta strain was first detected in the United States in March 2021. Since then, this strain has become the predominant strain in the U.S., accounting for 83% of cases. This strain appears to be faster and more infectious than the original Covid-19 strain.

Although some studies have shown that vaccines can prevent the delta strain, recent reports indicate that the delta strain can infect vaccinators at a higher rate than the previous strain. In addition, there is concern that vaccinators may spread the delta strain.

"For maximum protection against the delta variant and to prevent transmission to others, wear a mask indoors in public places if you are in a significant infected or highly infected area," suggests the CDC. Missouri, Mississippi, and Alabama currently have high infection rates.

According to CDC data, 46% of U.S. counties are currently "highly infected" and 17% are "significantly" infected. Furthermore, the CDC's latest guidance recommends that everyone in schools wear masks.

If you have a weakened immune system or an underlying medical condition, the CDC recommends wearing a face mask regardless of local infection levels. Also, if you are taking immune-compromising medications, you may not be protected against delta, even if you are vaccinated.